The present invention relates to semiconductor devices and methods of manufacturing the same. In particular, the present invention relates to a semiconductor device comprising elements, each having a photogate for converting light into an electric charge integrated with a transistor for transferring the electric charge. The elements are formed in a semiconductive layer which is deposited on an insulating layer to form a substrate, such as an SOI substrate, which is formed by depositing a silicon layer on an insulating layer. The present invention also relates to a method of manufacturing the semiconductor device and electronic apparatuses.
Elements, each having a structure in which a photogate and a MOS transistor are integrated with each other, have been recently used as a semiconductor device for transferring light as electrical signals. These elements are integrated on a bulk silicon substrate to form a semiconductor device in which the photogate converts absorbed light into an electric charge and in which the MOS transistor transfers the converted electric charge as an electrical signal. In this instance, in order to separate the elements, p-n junctions are formed in the silicon substrate and reverse bias voltage is applied to the p-n junctions to prevent current from flowing between the n-type areas.
In the case of an integrated circuit (IC) or a large-scale integration (LSI) circuit, transistors are densely integrated to highly integrate semiconductor elements; however, the elements directly formed in the bulk silicon substrate allow considerable junction leakage current to occur between a silicon layer and a source connected to the output circuit or between the silicon layer and a drain. The leakage current acts as dark current which causes noise, thereby lowering the S/N ratio and the dynamic range. Also, the elements directly formed in the bulk silicon substrate are likely to allow high-energy incident light to deeply penetrate the silicon layer, thereby causing an electron-hole pair. If this electric charge intrudes into the photogate adjoining thereto, no accurate correlation between the incident light and the electrical signal is exhibited.
The inventors have considered that a semiconductor device comprising elements formed in a semiconductive layer which is deposited on an insulating layer to form a substrate, such as an SOI substrate, which is formed by depositing an insulating layer and a silicon layer on a bulk silicon substrate in that order, would lead to a solution to those problems.
Specifically, it is expected that forming elements, each having the photogate integrated with the MOS transistor, allows the considerable leakage current to be lowered and prevents the electric charge caused by the high-energy incident light from intruding into the photogate, in the SOI substrate or the like.
Accordingly, it is an object of the present invention to provide a semiconductor device comprising elements, each having a photogate integrated with a MOS transistor, in a SIO substrate or the like, a method of manufacturing the same and electronic apparatuses.
To this end, a first semiconductor device of the present invention provides a semiconductor device comprising elements, each having a photogate for converting light into an electric charge integrated with a MOS transistor for transferring the electric charge. The elements are formed in a semiconductive layer which is deposited on an insulating layer to form a substrate. The thickness of the semiconductive layer forming the photogate is larger than that of the semiconductive layer forming the MOS transistor.
By forming the elements, each having a photogate integrated with a MOS transistor, in a semiconductive layer which is deposited on an insulating layer to form a substrate, such as an SOI, junction leakage current flowing between the source or the drain of the MOS transistor and the semiconductive layer is reduced, and consequently the S/N ratio and the dynamic range can be improved.
Also, by forming the semiconductor device in a semiconductive layer which is deposited on an insulating layer to form a substrate, electric charge generated by light is prevented from deeply penetrating the semiconductive layer, and consequently the electric charge can be efficiently collected.
Furthermore, by forming the semiconductive layer forming the photogate so as to have a thickness larger than that of the semiconductive layer serving as the MOS transistor, the semiconductive layer can have thicknesses suitable for the photogate and the MOS transistor.
Thus, the phototransformation efficiency of the photogate and the switching-speed performance of the MOS transistor are effectively improved.
The semiconductor device of the present invention may be formed by preparing elements in a semiconductive layer which is deposited on an insulating layer to form a substrate. Alternatively, the semiconductor device may be formed by preparing the elements in a semiconductive layer provided on another substrate, by removing the semiconductive layer from the substrate, and by putting the removed semiconductive layer on an insulating layer.
In the semiconductor device described above, the semiconductive layer forming the photogate may be in contact with the source area of the MOS transistor.
By forming the semiconductor device such that the semiconductive layer forming the photogate comes into contact with the source area of the MOS transistor, the electric charge converted by the photogate is quickly transferred to the source area of the MOS transistor, and thus the electric charge can be efficiently transferred.
Furthermore, the substrate of the semiconductor device described above may be an SOI substrate formed by depositing a silicon layer on an insulating layer.
A first manufacturing method of a semiconductor device according to the present invention is a method of forming a semiconductor device comprising elements, each having a photogate for converting light into an electric charge integrated with a MOS transistor for transferring the electric charge. The method comprises a step of preparing an SOI substrate formed by depositing a silicon layer on an insulating layer. A step of oxidizing the silicon layer deposited on the insulating layer is performed. In this step, the silicon layer is provided with an oxidation-resistant pattern thereon so as to cover the area for forming the photogate and to expose the area for forming the MOS transistor. The method also comprises a step of removing the silicon oxide layer formed by the oxidation. Thus, the area for forming the photogate has a silicon layer with a large thickness and the area for forming the MOS transistor has a silicon layer with a small thickness.
The method of manufacturing a semiconductor device includes the step of preparing an SOI substrate formed by depositing a silicon layer on an insulating layer, and the elements, each having a photogate integrated with a MOS transistor. As a result, semiconductor devices capable of effectively avoiding problems caused by forming elements on a bulk silicon substrate can be manufactured.
Also, the method includes steps of oxidizing the silicon layer which is deposited on the insulating layer and on which an oxidation-resistant pattern is provided so as to cover the area for forming the photogate and to expose the area for forming the MOS transistor and of removing the silicon oxide layer formed by the oxidation. Thus, the silicon layer can readily have different thicknesses so that the silicon layer has suitable thicknesses to serve as the photogate and the MOS transistor.
An electronic apparatus of the present invention comprises the semiconductor devices arranged in a matrix.
By arranging the semiconductor devices described above in a matrix, microscopic elements can be integrated on a plane surface, and thus the electronic apparatus can receive two-dimensional optical images.